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Native and Domestic Browsers and Grazers Reduce Fuels, Fire

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Native and Domestic Browsers and Grazers Reduce Fuels, Fire Ecological Applications, 24(4), 2014, pp. 741–749 Ó 2014 by the Ecological Society of America Native and domestic browsers and grazers reduce fuels, fire temperatures, and acacia ant mortality in an African savanna 1,2 2,3 2,4 2,5 2,6,7 DUNCAN M. KIMUYU, RYAN L. SENSENIG, CORINNA RIGINOS, KARI E. VEBLEN, AND TRUMAN P. YOUNG 1Department of Natural Resources, Karatina University, Karatina 10101 Kenya 2Mpala Research Centre, P.O. Box 555, Nanyuki 10400 Kenya 3Department of Biological Sciences, Goshen College, Goshen, Indiana 46526 USA 4Conservation Research Center, Teton Science Schools, Jackson, Wyoming 83001 USA 5Department of Wildland Resources, Utah State University, Logan, Utah 84322 USA 6Department of Plant Sciences, University of California, Davis, California 95616 USA Abstract. Despite the importance of fire and herbivory in structuring savanna systems, few replicated experiments have examined the interactive effects of herbivory and fire on plant dynamics. In addition, the effects of fire on associated ant–tree mutualisms have been largely unexplored. We carried out small controlled burns in each of 18 herbivore treatment plots of the Kenya Long-term Exclosure Experiment (KLEE), where experimentally excluding elephants has resulted in 42% greater tree densities. The KLEE design includes six different herbivore treatments that allowed us to examine how different combinations of megaherbivore wildlife, mesoherbivore wildlife, and cattle affect fire temperatures and subsequent loss of ant symbionts from Acacia trees. Before burning, we quantified herbaceous fuel loads and plant community composition. We tagged all trees, measured their height and basal diameter, and identified the resident ant species on each. We recorded weather conditions during the burns and used ceramic tiles painted with fire-sensitive paints to estimate fire temperatures at different heights and in different microsites (under vs. between trees). Across all treatments, fire temperatures were highest at 0–50 cm off the ground and hotter in the grass under trees than in the grassy areas between trees. Plots with more trees burned hotter than plots with fewer trees, perhaps because of greater fine woody debris. Plots grazed by wildlife and by cattle prior to burning had lower herbaceous fuel loads and experienced lower burn temperatures than ungrazed plots. Many trees lost their ant colonies during the burns. Ant survivorship differed by ant species and at the plot level was positively associated with previous herbivory (and lower fire temperatures). Across all treatments, ant colonies on taller trees were more likely to survive, but even some of the tallest trees lost their ant colonies. Our study marks a significant step in understanding the mechanisms that underlie the interactions between fire and herbivory in savanna ecosystems. Key words: Acacia drepanolobium; cattle; Crematogaster; elephants; herbivory; Kenya; Laikipia; livestock; mutualism; Tetraponera. INTRODUCTION of modeling approaches (van Langevelde 2003, Baxter Fire and herbivory are important drivers in savanna and Getz 2005, Holdo et al. 2009) and empirical studies systems, playing key roles in structuring tree–grass (Dublin et al. 1990, Moncrieff et al. 2008) have coexistence (reviewed in Sankaran et al. 2005, 2008), suggested that fire may interact with grazing and landscape heterogeneity (Fuhlendorf and Engle 2004, browsing in complex ways (Collins and Smith 2006) to Kerby et al. 2007, Waldram et al. 2008, Allred et al. create systems that are dynamic and nonequilibrial 2011), forage quantity and quality (Sensenig et al. 2010), (Bond and Keeley 2005), changing at various scales in and plant productivity (Holdo et al. 2009). Although it space and time (Gillson 2004, Archibald et al. 2005). is well documented that both fire (Mapiye et al. 2008, Understanding the mechanisms underlying how fire Levick et al. 2009, Winston and Trollope 2011) and and herbivory interact with changes in tree cover in herbivory (McNaughton et al. 1988, Levick et al. 2009) particular has both important theoretical and applied independently affect savanna ecosystems, there is implications, since variation in savanna woody cover increasing interest in understanding their potentially affects livestock production, wildlife conservation, interactive effects on tree and grass dynamics. A variety predator–prey interactions, nutrient cycling, and carbon storage (Scholes and Archer 1997, Angassa and Baars 2000, van Auken 2000, Bond and Keeley 2005, Riginos Manuscript received 17 June 2013; revised 16 August 2013; and Grace 2008). In particular, there is a lack of accepted 11 September 2013. Corresponding Editor: N. T. Hobbs. controlled replicated experimental studies that quantify 7 Corresponding author. E-mail: [email protected] (1) the separate and combined effects of domestic and 741 742 DUNCAN M. KIMUYU ET AL. Ecological Applications Vol. 24, No. 4 native herbivores on fuel loads and fire temperatures loads, (2) what are the patterns of ant colony survival and (2) the separate and combined effects of fire and after burning, and (3) do differences in ant colony subsequent herbivory on the survival of savanna trees. survival and subsequent shifts in ant species composition We address the former and describe the experimental or occupancy correlate with herbaceous fuel loads and design that will be used to test the latter. burn temperatures? A number of previous results have provided partial evidence for these interactions. For example, there is STUDY SITE AND METHODS some evidence from African savannas (Hobbs 1996, Study site and species O’Connor et al. 2011), North American grasslands (van This research was carried out in Acacia drepanolobium Auken 2000), and Australian savannas (Leonard et al. wooded savanna on the Laikipia Plateau, Kenya (368520 2010) that preburn grazing decreases fire frequencies and E, 08170 N; 1810 m above sea level). This plant intensities. This suggests feedback loops between her- community overlies high clay black cotton soils and is bivory and fire that current models of savanna dynamics representative of similar ecosystems that occur exten- do not include (Sankaran et al. 2004). However, most sively throughout eastern and southern Africa. Rainfall existing information on the relationship among herbi- averages 550–600 mm at our study site and is weakly vores, herbaceous fuel load, and fire behavior is based trimodal, with a distinct dry season from December to on anecdotal observations (Nader et al. 2007), studies March. involving only domesticated herbivores (Savadogo et al. The whistling thorn tree, Acacia drepanolobium, 2007, Gambiza et al. 2008, Davies et al. 2010), studies accounts for 97% of woody cover at our study site that do not control or monitor preburn grazing (Kerby (Young et al. 1998). At some branch nodes, A. et al. 2007), or those lacking replication (Leonard et al. drepanolobium produces hollow swollen thorns that 2010). serve as ant domatia. The trees also produce extrafloral Ant mutualisms may mediate fire–herbivore interac- nectaries at the bases of leaves to nourish symbiotic ants tions. Symbiotic ant species that reside on Acacia (Hocking 1970, Huntzinger et al. 2004). At our study drepanolobium trees are effective in reducing browsing site, each tree is occupied by one of four species of ants: by megaherbivores (Madden and Young 1992, Palmer et Crematogaster mimosae, C. sjostedti, C. nigriceps,or al. 2008a, Martins 2010, Stanton and Palmer 2011), Tetraponera penzigi. These ant species differ in various increasing individual tree fitness (Palmer and Brody aspects of their behavior, including the average size and 2007, Goheen and Palmer 2010, Palmer and Brody number of trees they occupy, their relative competitive 2013), and stabilizing tree cover across the landscape abilities, their benefits to host trees (Young et al. 1997, (Goheen and Palmer 2010). To the extent that fire may Stanton and Palmer 2011), and their behavioral weaken or destroy ant colonies (Palmer et al. 2008b), it responses to fire. During a fire, C. nigiceps and C. may lead to subsequent shifts in ant species composition mimosae evacuate domatia and take refuge in insulated or declines in ant abundance across the landscape, either cracks in the soil (Jaffe and Isbell 2009; T. Palmer, of which could make trees in burned areas more personal communication), reoccupying the tree after the susceptible to browsing and the destructive effects of fire. Tetraponera penzigi do not leave their domatia, and elephants, with subsequent cascades to other herbivore C. sjostedi take refuge in holes on tree stems, often and plant guilds. created by Cerambycidae larvae (Palmer et al. 2008a). In order to tease apart these complex dynamics The Kenya Long-term Exclosure Experiment (KLEE) among diverse herbivores (wild and domestic, grazing is located at the Mpala Research Centre. Since 1995, we and browsing), fire, savanna grasses, trees, and acacia have been manipulating the presence and absence of ants, we implemented prescribed burns inside the three guilds of large herbivores: livestock (cattle, C), replicated Kenya Long-term Exclosure Experiment wildlife (large mammals 15–1000 kg, W), and mega- (KLEE) in early 2013. By experimentally burning within herbivores (elephants and giraffes, M). There are 18 different herbivore treatments, we were able to simulta- plots, each 200 3 200 m, representing three replicate neously test the effects of a diverse guild of herbivores blocks of each of six combinations of large herbivores on woody and herbaceous
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